With 139 square miles of land within the city limits, and 60 percent fewer residents than in 1950, Detroit has 88,000 streetlights. To save $10 million a year the mayor has proposed borrowing $160 million to upgrade and reduce the number of streetlights to 46,000. Deciding which areas to illuminate and which to let go dark will reshape the city that has an estimated 37 square miles of vacant property and parks. Conventional wisdom is that state legislation is required to create the needed city lighting authority.